Gas-liquid mixing is adopted in various processes, typically in fermentation, waste water treatment, oxidation, hydrogenation, etc. Among those processes, aeration stirring can satisfy the required volume of oxygen for culturing in an aerobic fermentation process due to the aeration and stirring functions, but actually, in many cases of gas-liquid mixing, the productivity is decided by the oxygen supply capacity of the fermentation tank in use. The main object of the gas-liquid mixing will be to refine and disperse bubbles and absorb gas components into a liquid. As for gas absorption in a gas-liquid contact maker using a stirring tank, a well known relational expression (see Industrial Engineering Chemistry, volume 45, page 2554-2560 (1944)) is, as follows: EQU Kla.varies.Pv.sup..alpha..times.Us.sup..beta.,
wherein
KL is the mass transfer coefficient of liquid stirring; PA1 a is the gas-liquid interface are per unit volume; PA1 Pv is the stirring power per unit volume; PA1 Us is the superficial gas velocity; and PA1 .alpha., .beta. are constants.
In order to improve the efficiency of gas absorption, a problem of how to increase the gas-liquid interface area a, that is, how to minimize bubbles in size and disperse them, must be solved. Because, the KL in the above expression is decided by the solid state properties and fluid state of the material. Actually, however, the stirring power Pv and aeration capacity Us are increased to solve the problem.
In addition, in order to achieve the above object, measures had to be taken to refine bubbles efficiently while both stirring power and aeration capacity were suppressed from increasing as much as possible, as well as a more effective stirring blade unit had to be developed. And, in recent years, there are introduced a blade unit that can mix gas and liquid efficiently without damaging the microorganisms (Unexamined Published Japanese Patent Application No. 5-103956), a fermentation tank improving method that can improve the ferment shift capacity coefficient (KLa) by fixing a wire mesh in the target fermentation tank so as to surround the stirring blade unit(Examined Published Japanese Patent Application No. 3-4196), an effective method of mixing and gas-liquid contact by providing a gas inlet at the tip of the stirring blade unit (Examined Published Japanese Patent Application No. 57-60892), a stirring blade unit that can improve the stirring mixture effectively using a stirring blade unit that can rotate a pair of propellers and a perforated cylinder together (Unexamined Published Japanese Patent Application No. 6-85862), etc. and their effects are already confirmed.
Actually, however, it will be difficult to improve gas absorption by increasing the above stirring power and aeration capacity. Because the increase of those items is accompanied by expansion of the equipment and increasing of energy. When increasing the stirring power, measures such as increasing the rotation speed of stirring and increasing the blade size are thought of, but those measures will require improvement and reinforcement of some components related to the stirring, such as modification of the agitator itself, increasing of the strength of the stirring tank, etc. Especially, it will be difficult to apply such the improvement and reinforcement as mentioned above to existing equipment for the reasons of construction method and cost in many cases.
Furthermore, when operating any of those developed in recent years in an industrial scale, it will arise problems that the rotation speed must be more increased to obtain the expected effect, the equipment will become more complicated in structure, and the equipment will be more expanded in size (so that it cannot be fixed in the target stirring tank), etc. When the power characteristics of the blade unit differ from those of the conventional blade unit such as turbine blades, etc., the blades will be more expanded in size. In such a case, therefore, it will be difficult to apply the blade unit to any of existing stirring equipment.